The present application relates generally to fiber optic bundles contained within a stainless steel tube and in particular the desire to obtain a specified excess fiber length of the optic bundles from either or both extending ends of the tubes. More specifically, the present invention discloses a method with attached mathematical equations and special process techniques for section reducing the encasing stainless steel tube (i.e., decreasing gauge length concurrent with increasing gauge thickness) using straightening rollers in order to achieve the desired excess fiber length of the associated optic bundle and in order to provide sufficient tensile and compressive operating characteristics such as in aerial cable operations.
The prior art is well documented with examples of fiber optic tube feeding devices and processes, and those in particular in which a bundle of optic fibers are fed into an outer tube, such as is constructed of a stainless steel or other suitable environmentally insulating and/or electrically conductive jacketing material.
An objective in the manufacture of fiber optic bundles within their associated outer tubing is the creation of an excess length of the fiber strands at either or both terminating ends of the outer tubing. This is accomplished in one application by the overfeeding of the fibers into the tube, and such as again during the manufacture of the tube.
A shortcoming which has been determined to result from conventional overfeeding of the fiber bundles is that the bundle often does not exhibit sufficient tensile and compressive ratings for certain aerial cable applications, this often being due to an insufficient amount of excess fiber length (EFL) resulting from such conventional overfeeding applications. It has further been determined that, under extreme external loading cases, e.g. such as wind and ice, forces in the range of 60-80% of the rated breaking strength (RBS) of the cable are often achieved. For these particular aerial cable applications, it is required that the optical fibers in the cable are not strained at these heightened load conditions. The present invention process technique, using a system of straightening rollers, will make the cable to comply with this requirement, by providing an additional excess fiber length (EFL), not possible of obtaining through overfeeding techniques alone.
A further example of a process and apparatus for controlling the feed (overfeed) of a plurality of optical fibers within a tube, and while the tube is being formed, is illustrated in U.S. Pat. No. 4,640,576, issued to Eastwood. The process in Eastwood includes the steps of propelling a plurality of fibers into the tube with a sufficient force to move the fibers at a speed faster than the speed at which the tube is formed. Eastwood also teaches regulating the excess length of fiber by restraining the feed at a predetermined ratio with respect to the speed at which the tube is being formed.
Another example of an apparatus for installing optical fiber in a conduit is disclosed in U.S. Pat. No. 5,234,198, issued to Hale, and which teaches a ribbon of optical fiber introduced into a conduit (such as a duct) and including the use of a pressurized liquid transporting medium. The liquid transporting medium is effective to cause the optical fiber to be moved along in the conduit to cause a leading end of the fiber to emerge from a far end of the conduit and be accessible for connective arrangement.
The present invention discloses a novel method, with attached mathematical equations and a special process technique for section reducing an encasing stainless steel tube using a straightening roller assembly (i.e., decreasing gauge length concurrent with increasing gauge thickness) in order to achieve the desired excess fiber length of the associated optic bundle and in order to provide sufficient tensile and compressive operating characteristics such as in aerial cable operations. It is also understood that the tube gauge length decrease using the roller assembly may work in combination with existing and known overfeed techniques and in order to impart an extra desired degree of excess fiber length (EFL) than that which is possible through the use of overfeed techniques alone, while increasing a region of zero fiber strain established inside the tube and to thereby increase a maximum rated cable load under extreme loading conditions.
The present invention operates through the provision of either a single or, preferably, a pair of first and second planar shaped platforms arranged in adjoining and angular offsetting fashion relative to one another. Each of the platforms includes a first plurality, preferably first, second and third, of stationary mounted and spaced apart rollers. A second plurality, preferably first and second, adjustable rollers are likewise mounted in linear and spaced apart fashion and so that they oppose the first plurality of stationary rollers of each associated platform.
The further advantage of employing first and second platforms is so that appropriately configured and recessed grooves, associated with each plurality of rollers of a designated platform, contacts a selected cross sectional location of the tube (along its axial extending length). In combination, the rollers apply a lateral contact pressure along the tube in order to decrease a gauge length associated with the tube, while correspondingly increasing a gauge thickness of the tube to thereby cause excess fiber to extend from either or both ends of the tube.
Additional features include the provision of a coil biasing spring for each adjustable roller and which ideally seats within a channel defined in the associated platform and in perpendicular extending fashion relative to the linear travel direction of the tube and fiber bundle. An adjustment screw is also provided for each biasing spring and, upon rotation in a specified direction, establishes a desired contacting position and biasing force of the roller. As also explained previously, a single plane roller assembly can also be utilized by itself and within the scope of the invention.
The additional increase in excess fiber length (EFL) is due to the fact that, even if the fiber length remains the same within a xe2x80x9cgaugexe2x80x9d length, the xe2x80x9cgaugexe2x80x9d length of the tube will decrease, after going through the system of straightening rollers, from an xe2x80x9cinitialxe2x80x9d value, in front of the rollers, to a shorter xe2x80x9cfinalxe2x80x9d value, after the rollers.
The range of additional excess fiber length (EFL), in percentage, that can be obtained through this novel technique, is controlled by a range of lateral deformation, in tenths of millimeters, that is applied to the tube. The lateral deformation range is controlled by a range of lateral contact pressure applied on the tube by the rollers, through the coil spring system.
The physical explanation for this phenomenon is that the system of straightening rollers, through the lateral pressure exercised, gradually, up to 360 degrees, all the way around the tube, will determine a small reduction of the material in the axial direction, in xe2x80x9clengthxe2x80x9d, and a very small increase in the transverse direction, in xe2x80x9cthicknessxe2x80x9d. The small reduction in xe2x80x9clengthxe2x80x9d of the tube is transferred in an very small increase in xe2x80x9cthicknessxe2x80x9d of the tube. When the tube travels first, through the system of vertical rollers, there is, first, a lateral contact pressure exercised on 2 points: top (xe2x80x9cNorthxe2x80x9d) and bottom (xe2x80x9cSouthxe2x80x9d). When the tube, subsequently, travels through the system of horizontal rollers, a lateral contact pressure is exercised on another 2 points: xe2x80x9cWestxe2x80x9d and xe2x80x9cEastxe2x80x9d, on the 2 lateral sides of the tube. In the end, after going through both the vertical and horizontal system of straightening rollers, the tube will be under lateral contact pressure, in 4 points, located all the way around its circumference, 90 degrees apart.
Both the reduction in the axial direction and the increase in the transverse direction are in the order of a few tens of a percent. In absolute values, the reduction in the axial direction represents a few millimeters in 1 m gauge length, while the increase in the traverse direction represents a few (such as 1-9) tenths of a micron.
This novel technique of obtaining excess fiber length (EFL) in steel tubes will determine a substantial increase in fiber tensile window (region without fiber strain) inside the steel tube, and it will determine a substantial increase in the value of Cable Tension with Zero Fiber Strain (CTZFS) and in the value of Maximum Rated Cable Load (MRCL), under extreme external loading cases: wind and ice, required in some specific aerial cable applications.
This novel technique of obtaining excess fiber length (EFL) in steel tubes will determine also a high extension in the application possibilities of the optical groundwire design with just one xe2x80x9ccentralxe2x80x9d tube, also called xe2x80x9cunitubexe2x80x9d design, instead of using the more expensive xe2x80x9cstrandedxe2x80x9d tube designs, in which the necessary fiber tensile window (region without fiber strain) is obtained using the lay length (the pitch) of the tube.